<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>position</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        .div_body_main {
            width: 100%;
            height: 4043px;
            background: #85e0ff center top no-repeat;
        }

        .div_nav_main {
            width: 160px;
            height: 205px;
            position: fixed;
            left: 0;
            top: 50%;
            margin-top: -103px;
            font-family: 'Miscrosoft YaHei';
        }

        .item_div_一级栏目 {
            width: 160px;
            height: auto;
            border-bottom: 1px solid #FFF;
            background: #333;
            text-align: center;
            line-height: 40px;
            color: #FFF;
            font-size: 16px;
            cursor: pointer;
        }

        .title {
            width: 160px;
            height: 40px;
        }

        .item_div_一级栏目:hover ul {
            display: block;
        }

        .item_div_一级栏目 ul {
            width: 160px;
            height: auto;
            background: #FFF;
            display: none;
        }

        .item_li_二级栏目 {
            width: 160px;
            height: 40px;
            border-bottom: 1px dashed #666;/*画虚线*/
            color: #333;
            text-align: center;
            line-height: 40px;
            position: relative;
        }

        .item_li_二级栏目:hover .div_三级栏目 {
            display: block; /*显示*/
        }

        .div_三级栏目 {
            width: 160px;
            height: auto;
            position: absolute;
            left: 160px;
            top: 0px;
            display: none; /*不显示*/
        }

        .item_三级栏目 {
            width: 160px;
            height: 40px;
            background: #444;
            border-bottom: 1px solid #FFF;
            text-align: center;
            line-height: 40px;
            color: #FFF;
        }

    </style>
</head>
<body>
<div class="div_body_main">
    <div class="div_nav_main">
        <div class="item_div_一级栏目">
            <div class="title">一级栏目</div>
            <ul>
                <li class="item_li_二级栏目">
                    二级栏目
                    <div class="div_三级栏目">
                        <div class="item_三级栏目">三级栏目</div>
                        <div class="item_三级栏目">三级栏目</div>
                        <div class="item_三级栏目">三级栏目</div>
                    </div>
                </li>
            </ul>
        </div>
        <div class="item_div_一级栏目">
            <div class="title">一级栏目</div>
            <ul>
                <li class="item_li_二级栏目">
                    二级栏目
                    <div class="div_三级栏目">
                        <div class="item_三级栏目">三级栏目</div>
                        <div class="item_三级栏目">三级栏目</div>
                        <div class="item_三级栏目">三级栏目</div>
                    </div>
                </li>
            </ul>
        </div>
    </div>
</div>

</body>
</html>